diff --git a/kernel/src/vm/vmar/vmar_impls/protect.rs b/kernel/src/vm/vmar/vmar_impls/protect.rs
index 231241544..d85f3dfff 100644
--- a/kernel/src/vm/vmar/vmar_impls/protect.rs
+++ b/kernel/src/vm/vmar/vmar_impls/protect.rs
@@ -43,7 +43,12 @@ impl Vmar {
             let new_perms = perms | (vm_mapping_perms & VmPerms::ALL_MAY_PERMS);
             new_perms.check()?;
 
-            let vm_mapping = inner.remove(&vm_mapping_range.start).unwrap();
+            let Some(vm_mapping) = inner.remove(&vm_mapping_range.start) else {
+                // This can happen only if the mapping is merged to the previous one (just
+                // protected before). We can skip this mapping because its property is already
+                // correct.
+                continue;
+            };
             let vm_mapping_range = vm_mapping.range();
             let intersected_range = get_intersected_range(&range, &vm_mapping_range);
 
diff --git a/test/initramfs/src/apps/mmap/mmap_and_mprotect.c b/test/initramfs/src/apps/mmap/mmap_and_mprotect.c
index 50021e99c..679095ccf 100644
--- a/test/initramfs/src/apps/mmap/mmap_and_mprotect.c
+++ b/test/initramfs/src/apps/mmap/mmap_and_mprotect.c
@@ -69,3 +69,17 @@ FN_TEST(mprotect_invalid_permission)
 	TEST_SUCC(munmap(addr, PAGE_SIZE));
 }
 END_TEST()
+
+FN_TEST(mprotect_merge_after_split_mapping)
+{
+	void *addr = TEST_SUCC(mmap(NULL, 2 * PAGE_SIZE, PROT_READ | PROT_WRITE,
+				    MAP_PRIVATE | MAP_ANONYMOUS, -1, 0));
+
+	// Split the mapping into two.
+	TEST_SUCC(mprotect(addr, PAGE_SIZE, PROT_READ));
+	// After the first mapping is protected, the two mappings will merge.
+	TEST_SUCC(mprotect(addr, 2 * PAGE_SIZE, PROT_READ | PROT_WRITE));
+
+	TEST_SUCC(munmap(addr, 2 * PAGE_SIZE));
+}
+END_TEST()
\ No newline at end of file